Firn lines, like end of summer snow lines, are used as a proxy of glacier mass balance. The snow line, or long term firn line, is related to the Equilibrium Line Altitude (ELA), a useful climate indicator. Mapping the position of the ELA in situ is difficult and expensive given the inaccessibility of many glaciers and ice sheets. Satellite data has been used to map snow line for decades: since the launch of ERS-1 in 1991 synthetic aperture radar (SAR) images, that penetrate clouds, have been used to map summer snow lines. SAR images have also been used to map firn lines in winter. The SAR penetrates dry snow and backscatters from firn. Firn appears very bright, and glacier ice below the ELA much darker (typically 6-12 dB darker). One problem has been that it is difficult to infer changes in mass balance from firn observations. A positive mass balance might result in firn line advance, detectable in SAR data. But negative mass balances may not result in firn line retreat, just downwasting (melting of the upper layers of firn).
In an article accepted for publication in the IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing (JSTARS) I show for the first time, that it is possible to identify a distinct radiometric signal from retreating firn. Hence, in some cases, we can make inferences of glacier mass balance from firn observations in SAR data.
Abstract—The firn line, like other glacier facies, is mapped operationally as part of glacier monitoring activities for glaciological and climate studies. Synthetic aperture radar (SAR) images are commonly used to determine the firn line in dry snow imagery. The radiometric response of retreating firn has not previously been investigated. Rather, it has been assumed that firn line mapping is only useful where large scale advances or retreats of the lower limit of continuous firn have occurred. In this paper the radiometric signal of retreating firn on an icecap in north Norway is analyzed using multi-temporal SAR imagery. Using comparisons with firn well above the firn line and field investigations of the firn properties, backscattering mechanisms are inferred. It is found that retreating firn has a distinctive radiometric signal that can be used to identify the inception and progression of firn down-wasting prior to and during firn line retreat.
Index Terms—Synthetic aperture radar, Scattering, Ice, Hydrology